1. Field of the Invention
The present invention relates to the production of an improved foam of a thermosettable phenol-formaldehyde condensate. More particularly, it relates to an improved phenolic foam useful for insulating metallic conduits and other substrates which has a high resistance to combustion, is anti-punking, and is of low corrosion to the metallic substrate.
2. Description of the Prior Art
With the advent of reinforced plastic casting, laminating and molding technology, the use of plastic materials has grown to include structural and decorative applications in buildings, aircraft, vehicles and other structures. The majority of the economically useful plastics are, however, combustible and the flammability of such materials is a prime consideration in determining their degree of usefulness in a given application.
Phenolic resin foams are noted for their low flammability and their ability to resist a direct application of flame. These materials do not melt or soften unless such flame is accompanied by copious amounts of air or oxygen. It has been known to prepare such foams from aqueous phenol-formaldehyde resins using an acid catalyst. The reaction is exothermic, which converts the water present to steam. The liquid resin is gradually converted into an infusible solid, entrapping such steam which, in turn, gives the cellular structure of the foam. In place of, or in conjunction with water, other volatiles may also be used to aid in foam formation. However, it is a well-recognized problem that the phenolic foams produced in this manner experience severe punking after being exposed to flame. Punking, as is well known, is the phenomenon of continuing to glow and combust without a visible flame even after the combustion source has been removed. Such punking is a serious limitation in the use of these materials as thermal insulation, especially in inhabited structures.
The production of non-punking foams has generally been disclosed in U.S. Pat. No. 3,298,973. In that patent, phenol-aldehyde resole resins having a viscosity of from about 200 to about 300,000 centipoises at 25.degree. C. are reacted with a catalyst that is a mixture of at least two acidic reagents. The catalyst is a solid mixture of boric acid or its anhydride and an organic hydroxy acid in which the hydroxy group is on a carbon atom not more than one carbon atom removed from a carboxy group. Because of the viscosity of the resole resin, this process is disadvantageously carried out by a slow and tedious batch procedure. Further, even when using large amounts of boron oxides in these foams, punking is not completely controlled.
Other boron-containing, non-punking phenol-formaldehyde foams are described in U.S. Pat. No. 3,663,489 disclosing boron-containing compounds formed by reacting boric acid or boric oxide with glyoxal and its derivatives, and U.S. Pat. No. 3,740,358 disclosing boron-containing compositions utilizing boric acid or boric oxide in conjunction with hydrochloric acid. British Pat. No. 824,251 sets forth a method for the production of phenol-aldehyde castings free from voids by using as catalysts boric acid or boric oxide with hydroxy organic compounds. The catalysts described in this latter patent are not capable of producing foams, and there is no indication that the materials produced have any improved fire-retardant or heat-resistant properties. The latter two United States patents, while producing non-punking foams, result in foamed materials that are extremely high in acidity and, when in contact with metallic substrates such as metallic pipes, girders, panels, tubes, and the like, cause severe corrosion.
It is apparent, therefore, that there is a need in industry to provide heat-resistant phenolic foams which have a high degree of anti-punking character and which are also non-corrosive to metallic substrates over or on which such may be applied.